docs: def mean_size_ISO(self, k, r):
          '''        
          >>> psd = PSDLognormal(s=0.5, d_characteristic=5E-6)
          >>> psd.mean_size_ISO(1, 2)
          4.412484512922977e-06
          '''
          p = k + r
          q = r
          return self.mean_size(p=p, q=q)